1. Field of the Invention
The present invention relates to a data coding apparatus and data coding method for audio signal such as voice signal and music signal. More particularly, it relates to a data coding apparatus generating a frequency conversion signal by converting from time domain into frequency domain by using quadrature conversion or other technique, compressing the data by a smaller coding quantity, and generating a encoded bit stream at high speed in order to express at high sound quality.
2. Discussion of the Related Art
Many systems have been developed hitherto as audio signal data coding methods, and some are put in actual use. There are two types of data coding methods for audio signal, namely a fixed bit rate method and a variable bit rate method. ADPCM and Dolby-AC3 are known as the fixed bit rate methods. An advanced audio coding, hereinafter refers to as an AAC, is known as the variable bit rate coding. The AAC is internationally standardized by the ISO/IEC in the IS13818-7, and evaluated as a data coding method of high sound quality and high efficiency. The IS13818-7 of the ISO/IEC is the standard of decoding process and coding format, but the data coding method itself is not designated. In the AAC, signal of each frequency band is compressed on different compressing parameters not on a common parameter. And high sound quality and high efficiency can be obtained based on proper compressing parameters, in each frequency band.
However, in the AAC, inadequate parameter selection causes reduce of compressing rate, or lack in spectrums. And the lack in spectrum frequency causes much distortion when reproducing the coded signal. Furthermore, it requires much processing power when coding in the AAC system and it is difficult for audio and video apparatus of home appliance to encode input signal on the real time basis.
The invention is to solve the problems of the conventional method. It is an object of the invention to provide a data coding apparatus and data coding method which decrease the quantity of calculation, and hence issue the encoded bit stream at high speed. It is also an object to decode the audio signal optimally so that the reproduced sound may have a high sound quality.
The present invention is a data coding apparatus for quantizing and coding an audio signal divided into plural frequency bands at a quantizing precision determined in each frequency band, and comprises a relative quantizing precision decision unit, a quantizing precision guarantee value decision unit, a quantizing processing unit, and a coding unit.
The relative quantizing precision decision unit calculates relative quantizing precision information used in quantizing from representative frequency information, where the representative frequency information is a representative or maximum value of the amplitude of each frequency band, and the relative quantizing precision information is quantizing precision information relative to the amplitude of each frequency band.
The quantizing precision guarantee value decision unit calculates a guarantee value of quantizing precision information which is the quantizing precision information to have a minimum assured amplitude when decoding in each frequency band.
The quantizing processing unit quantizes at a quantizing precision of each frequency band that is higher than or equal to the quantizing precision indicated by the guarantee value of the quantizing precision information, by using the output value of the relative quantizing precision decision unit and the output value of the quantizing precision guarantee value decision unit.
The coding unit codes the information quantized in the quantizing processing unit, and generates audio coded data, and the quantizing processing unit quantizes so that the coded data from the coding unit is less than or equal to the predetermined information quantity.